260 Watson: Discoloration and Decay in Severed Tree Roots Arboriculture & Urban Forestry 2008. 34(4):260–264. Discoloration and Decay in Severed Tree Roots Gary Watson Abstract. Roots of honeylocust (Gleditsia triacanthos var. inermis), pin oak (Quercus palustris), tuliptree (Liriodendron tulip- ifera), and green ash (Fraxinus pennsylvanica) trees were severed at the root flare and 1, 2, or 3 m (3.3, 6.6, and 9.9 ft) from the trunk. After 5 years, the severed roots were excavated and all discolored and decayed portions were removed. The furthest extent of decay development ranged between 4.5 cm (1.8 in) in green ash and 10.8 cm (4.3 in) in honeylocust. The furthest extent of discoloration also varied between 6.3 cm (2.5 in) in green ash and 77.1 cm (30.8 in) in honeylocust. The root severing location producing the greatest decay or discoloration varied among species. Natural defect development as a result of severing roots of any size root at any distance is not likely to result in a threat to the health or stability of a tree. Key Words. Compartmentalization; defect; Fraxinus pennsylvanica; Gleditsia triacanthos var. inermis; Liriodendron tulipif- era; Quercus palustris; root severing. Root injury is common in urban trees. Nursery production, trans- planting, construction activity, and utility installation frequently result in root injury. Introduction of extensive discoloration and decay defects through injury to tree trunks and branches is well known by arborists. This awareness has led to widespread con- cern that similar mechanical root injury can introduce extensive decay into tree root systems and that it could eventually spread to the trunk and lead to major tree health and stability problems. Even in nature, without human activity, root injury is com- mon. Formation of lateral roots creates injury when outer layers of cells (endodermis, cortex, and epidermis) are ruptured as the new root emerges from the pericycle layer in the interior of the root where it is initiated (Esau 1960). Insects, compression, ani- mal trampling, excessive moisture, abrasion against stone sur- faces, breakage of small laterals by movement resulting from frost heaving and the root plate rocking in the wind, and dieback response to root stress or root disease all produce injuries to roots (Redmond 1957; Whitney 1961; Stone 1977; James et al. 1980). Root dieback can also result from crown stresses such as defo- liation (Redmond 1957). When secondary roots die, they leave natural openings that can act as infection courts for decay fungi (Redmond 1957; Shigo 1979b; Robinson and Morrison 2001). Discoloration and decay are the principal defects associated with injuries to trees (Shigo 1991). Wood discoloration is caused by invasion of bacteria and nonhymenomycetous fungi. The dis- colored wood area is often larger than pathogen-colonized area (Shigo and Hillis 1973; Garboletto et al. 1997). Compartmentalization Of Decay In Trees (CODIT; Shigo 1977) principles apply to roots as well as stems (Shigo 1972; Shigo 1979b; Tippett and Shigo 1981; White and Kile 1993; Robinson and Morrison 2001), although roots have not been as extensively studied. Because root injuries are common, and injuries serve as infection courts for root-rotting organisms (Tippett et al. 1982), roots have evolved to be strong compart- mentalizers (Shigo 1986). Dead roots are of less significance than dead tops in providing courts of entry for decay (Redmond 1957). There is evidence that decay of root wood blocks is greater than stems in laboratory tests and was attributed to higher nitro- ©2008 International Society of Arboriculture gen and carbohydrates (Platt et al. 1965). Rate of decay devel- opment in live roots after inoculation with root rot fungi is slower than in stems and may reflect the higher proportion of living cells in roots (White and Kile 1993). Literature reports on rates of root wood decay development are dominated by inoculation experiments with aggressive root rotting fungi such as Heterobasidium annosum and Armillaria mellea on commercially valuable conifers. In nature, it is un- likely for wood to be decomposed completely by one organism (Shigo 1967). Average values of longitudinal extension of decay columns after inoculation have been reported from 10 to 53 cm/yr–1 (4 to 21 in/yr–1) (White and Kile 1993; Morrison and Redfern 1994; Garboletto et al. 1997; Piri 1998). Colonization often progresses faster in the proximal direction (toward the stem) from the wound (Yokota 1962; Shigo 1979b; Garboletto et al. 1997). Decay introduced experimentally through root wounds within a meter of the trunk can extend into the trunk (Redmond 1957; Garboletto et al. 1997). In one report, trunk wood discoloration and decay were observed only when the root cambium had died back to, or above, the soil surface (Santamour 1985). Colonization rate can be increased by drought stress (Towers and Stambaugh 1968; Lindberg and Johansson 1992). Fertiliza- tion has been reported to both increase (Piri 1998) and decrease (Singh 1983) decay fungal colonization. Stimulation or inhibi- tion of decay fungus colonization by stress seems to be depen- dent on level of the stress factor as well as host and pathogen species (Wahlstrom and Barklund 1994; Desprez-Loustau et al. 2006). Root size and proximity to the trunk has been reported to affect decay development rate. Root decay increased as root size increased on hardwoods (Whitney 1967; Santamour 1985; Bal- der et al. 1995) and conifers (Garboletto et al. 1997; Piri 1998; Tian and Ostrofsky 2007). Injury to roots closer to the trunk resulted in more extensive defects on hardwoods (Balder et al. 1995). In the only previous study of defect development after wound- ing roots of landscape trees, roots of 7-year-old sweetgum (Li- quidambar orientalis × L. styraciflua) and plane hybrids (Plata-
July 2008
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